1. Field of the Invention
The present invention relates to an RF angle rate interferometer.
2. Description of Related Art
U.S. Pat. No. 3,090,957 to Albanese et al. describes an aircraft guiding system for landing aircraft at airports that uses a phase rate interferometer homing system based upon signals received by two spaced apart antennas on an aircraft. U.S. Pat. No. 3,789,410 to Smith et al. describes a passive ranging technique based upon a rate of change of a phase difference between two signals received by widely separated pairs of antennas on an aircraft. U.S. Pat. No. 4,704,613 to Albanese et al. describes a phase-rate interferometer passive ranging system based upon signals received at two spaced apart antennas on an aircraft. All of these known systems rely on a significantly long observation interval.
When attempting to range on an emitting radar, the radar is seldom emitting a steady beacon for a significantly long observation interval. In fact, often such emitting radars are emitting for only a short time duration to avoid detection or other reasons. The short time duration is in the order of a hundred milliseconds or so. As will be discussed herein, the frequency difference observed by two spaced apart antennas at desired ranges is often in the order of only a few Hertz in the desired ranges (about 20 kilometers). To achieve desired range accuracies (about 20% of range), it is necessary to not only measure the frequency difference, but also to measure the frequency difference very accurately. An improved approach is needed to achieve accurate enough measurements of phase rate in just a hundred milliseconds or less.